Waveguides are commonly referred to as “acoustical transformers” transforming the acoustical impedance from a horn input to the compression driver output. The current invention is to be implemented in line array systems as a transition element between the compression driver output and the high frequency line-array input, usually a rectangular vertical area band, very narrow in a horizontal plane, which makes possible fast horn flares opening, thus defining a relatively wide horizontal coverage. The vertical directivity of a line array system is typically realized by aligning such horns as close as possible to each other in a vertical line or in a slightly curved line, in both cases trying to simulate a cylindrical or prolate spheroidal wave front of the line array group up to the highest audible frequencies. To achieve this, all individual wave front outputs must be in-phase, all the way from top to bottom along its height, in order to create a coherent common wave front, without the typical for the conical horn groups vertical lobbing. Prior art teaches us how to do this in several ways.
In U.S. Pat. No. 5,163,167/Nov. 10, 1992, Heil teaches us how to build “Sound Wave Guide”, shown in FIG. 1—“Prior Art”, comprising a conduit which expands from its input to its output. The area of the output orifice of the wave guide is planar and oblong, and its conduit comprises a passage between the input orifice and the output area, adapted to guide the waves along a general direction from which the shortest paths allowed in the one or more passages are all of lengths which are practically identical from the input orifice to the output orifice of the conduit. This “Sound Wave Guide” is well-accepted and is in production by L-Acoustic Company, used in so-called V-DOSC Systems. One of the disadvantages in this prior art example is that expanding in axial direction in front of the driver, the length of the waveguide becomes relatively large. Another, probably worse, disadvantage is that this axial expansion actually widens the air-passages along the way towards the middle of the guide, where the wave front is forced to change direction to the rectangular output. With larger wall to wall distances at these foldings, inevitable phase interferences take part at higher frequencies, having quarter of the wavelengths comparable with these distances.
Adamson, in U.S. Pat. No. 6,581,719 B2/Jun. 24, 2003 teaches us how to use a “Wave Shaping Sound Chamber” with approximately rectangular inlet and outlet of substantially the same size in front of a typical conical horn throat. The sound chamber transforms the curvature of the fan shaped wave front that results from a conical horn throat into a wave front that approximates a planar or curved rectangular ribbon of sound. The invention claims advantages against the first prior art example, but at the expense of higher complexity, larger dimensions and eventually greater total line-array volume and mass.
In PCT published patent WO 2012/018735 A1, Donarski teaches us how to realize a plane wave front at the waveguide rectangular output by using two successive waveguides, the first being conical from the driver circular output to an annular output and the second with an annular input to a rectangular output. In both waveguides, vanes are used to address the interference problems at higher frequencies because of the increased dimensions of the air passages. Just as the previous prior art examples, this approach leads to very long, complicated and expensive waveguides, further increasing the length of the single line array element and the volume and the mass of the entire line array group.
None of the examples quoted, nor the art, to our knowledge, teach us how to make compact waveguides with precise wave front control up to the highest audible frequency range, having the possibility to precisely keep a predetermined wave front curvature at different output heights, expansion rates and axial lengths, at the same time being equally suitable for all the variety of available compression drivers. What is neither disclosed nor suggested in the art, is an acoustic waveguide that does not have the problems and limitations of waveguides in prior art, as described above.